Tips on planting dahlia tubers

Originally published April 25, 2012 at 3:00 pm

Updated April 25, 2012 at 6:16 pm

Ciscoe Morris, Seattle Times garden writer, offers tips on planting dahlia tubers and avoiding blossom end rot on tomatoes. He also talks about the effectiveness of using corn gluten in our area as a natural product that will keep chickweed and other low-growing weeds from taking over vegetable and ornamental flower beds.

Q: I bought a number of dahlia tubers recently. When and how should I plant them?

A: Dahlia tubers are best planted after the soil has warmed a bit, between mid-April and mid-May. In a sunny location with soil that drains well, dig a deep, wide hole and work in a good amount of compost. Dahlias are big feeders, so mix in a half-handful of bone meal and a cup of organic flower food along with the compost.

Plant the tuber lying horizontally with the growing point, or eye, facing upward. The tubers of large-growing dahlias should go about 6 inches deep, while smaller-growing ones are better planted 4 inches deep.

If the dahlia you’re planting will grow to more than 2 feet tall, pound a stake in the hole and lay the tuber with the eye end about 2 inches from the stake. That way, you won’t skewer the tuber when you decide to stake it. Bait for slugs as soon as growth appears, but wait to water until the new growth reaches 6 inches high. After that, water regularly to keep the soil evenly moist at all times.

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Mulch an inch thick with compost to keep the roots cool and reduce evaporation.

Every six weeks, work a mixture of 2 cups of alfalfa meal and about a cup or organic flower food around the drip line of the plant. Then, as long as you deadhead spent blossoms often, your dahlia should pump out spectacular blooms all summer long.

Q: Last year, the bottoms of my tomatoes turned black. How can I prevent this?

A: This is a common and frustrating problem known as “blossom end rot.” Symptoms begin with a water-soaked spot that quickly expands and often turns the entire bottom half of the tomato black and leathery. This isn’t a disease. It’s a physiological problem caused by a lack of calcium. Most of our soils in Western Washington are naturally deficient in calcium, so it’s always a good idea to incorporate agricultural lime into the soil of your vegetable garden in the fall to give it time to break down into a form usable by the plants by spring.

If you didn’t incorporate lime in fall, mixing a handful of calcium-rich bone meal into the soil under each tomato at planting time can help. Even with adequate amounts of calcium available, however, the plant won’t be able to absorb it if the soil is allowed to become too dry between waterings, or if a salt buildup occurs from using too much synthetic fertilizer.

Keep the soil evenly moist and fertilize only once by mixing in a cup of organic vegetable food at planting time. If it happens this year, don’t toss the whole tomato. Cut off the black part and eat the rest, because the taste won’t be affected.

Q: Is it true that corn gluten is a natural product that will keep chickweed and other low-growing weeds from taking over my vegetable and ornamental flower beds?

A: While looking for ways to make use of corn gluten, a byproduct of the corn-milling process, scientists at Iowa State University discovered that spreading it on the soil killed weed seeds as they germinated by drying them out in a critical stage of the process. At the same time, they also discovered that corn gluten is beneficial to existing plants by acting as a potent fertilizer.

Not surprisingly, corn gluten was patented and is now available as a popular environmentally friendly alternative to using chemical pre-emergent weed controls.

Unfortunately, in our Puget Sound climate, corn gluten has not turned out to be the silver bullet many of us hoped it would be. Researchers at Oregon State University found that corn gluten failed to prevent weed-seed germination in our area of the Pacific Northwest.

Evidently our moist spring weather allows the seeds to overcome the drying effect. The seedlings not only survive, but they thrive and get big and strong, thanks to all of that additional fertilizer. So, if you’re looking for a good natural fertilizer, corn gluten fits the bill, but at least in spring it’s not a good choice for weed control in our area.